Respiration monitor for computed tomography

ABSTRACT

A medical diagnostic imaging system ( 10 ) includes a diagnostic imaging scanner ( 12 ) that acquires imaging data of a medical imaging patient. A reconstruction processor ( 46 ) reconstructs at least a portion of the acquired imaging data into an image representation. A pair of electrodes ( 30, 32 ) contact a thoracic region of the patient. An electrical meter ( 34 ) measures electrical resistance R(t) or another time-varying electrical parameter ( 70 ) across the electrode pair ( 30, 32 ) during the acquiring of imaging data. A respiration monitor ( 36 ) extracts a time-varying respiration characteristic ( 90, 98, 110, 120 ) from the measured time-varying electrical parameter ( 70 ) to indicate respiratory cycle phase.

The following relates to the diagnostic imaging arts. It findsparticular application in respiratory compensated or gated diagnosticimaging, and will be described with particular reference thereto.However, it will also find application in conjunction with variousdiagnostic imaging modalities, such as computed tomography, singlephoton emission computed tomography, positron emission tomography,magnetic resonance imaging, and the like.

In situ monitoring of patient respiration during medical diagnosticimaging advantageously facilitates image registration over therespiratory cycle, sorting or binning of acquired imaging data withrespect to respiratory state, data filtering based on respiratory state,prospective respiratory gating, radiation dose modulation synchronizedwith the respiratory cycle, and the like. Respiratory monitoring canalso be useful for automating imaging. For example, initiation ofimaging can be triggered by detection of the start of a patientbreath-hold.

Heretofore, patient respiration has been monitored by electromechanicaltransducers that monitor chest movement. These electromechanical devicesare relatively bulky, can feel uncomfortably restrictive to the patient,and may inhibit chest movement and respiration. In one example, an airfilled tube is wrapped around the patient's torso. Inhaling increasespressure in the tube, which is sensed by an associated pressuretransducer.

Moreover, electromechanical respiration monitors can interfere withimaging, depending upon the transducer materials and the imagingmodality employed. Fitting the patient with an electromechanicaltransducer is time-consuming and adds complexity to the preparatoryphase of the imaging session. In the case of cardiac or other types ofimaging in which both the respiratory cycle and the cardiac cycle aremonitored, preparation for imaging includes fitting the patient withboth the electromechanical respiration monitor and electrodes of anelectrocardiograph.

The present invention contemplates an improved apparatus and method thatovercomes the aforementioned limitations and others.

According to one aspect, a diagnostic imaging system is disclosed. Adiagnostic imaging scanner acquires imaging data of a subject in anexamination region. A reconstruction processor reconstructs the acquiredimaging data into an image representation. A pair of electrodes areadapted to contact a thoracic region of the subject. An electrical metermeasures a time-varying electrical parameter across the electrode pair.A monitor extracts a time-varying respiration characteristic from themeasured time-varying electrical parameter.

According to another aspect, a medical diagnostic imaging method isprovided. Imaging data of a medical imaging patient is acquired. Atleast a part of the acquired imaging data is reconstructed into an imagerepresentation. A time-varying electrical parameter is measured acrossan electrodes pair during the acquiring of imaging data. A time-varyingrespiration characteristic is computed based on the measuredtime-varying electrical parameter.

One advantage resides in providing respiratory gated medical diagnosticimaging without encumbering the imaging patient with a bulky anduncomfortable electromechanical transducer.

Another advantage resides in providing an electronic signal indicativeof aspects of respiratory activity such as inhalation or a breath hold.

Yet another advantage resides in providing respiration informationduring cardiac computed tomography without contacting the patient withadditional probes.

Numerous additional advantages and benefits will become apparent tothose of ordinary skill in the art upon reading the following detaileddescription of the preferred embodiments.

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for the purpose ofillustrating preferred embodiments and are not to be construed aslimiting the invention.

FIG. 1 diagrammatically shows a computed tomography imaging systemincluding in situ respiratory cycle monitoring.

FIG. 2 diagrammatically shows a suitable embodiment of the impedancemeter of the computed tomography imaging system of FIG. 1.

FIG. 3 diagrammatically shows a suitable embodiment of the respiratorymonitor of the computed tomography imaging system of FIG. 1.

FIG. 4 diagrammatically shows another suitable embodiment of therespiratory monitor of the computed tomography imaging system of FIG. 1.

With reference to FIG. 1, a computed tomography imaging system 10includes a computed tomography scanner 12. Although the invention isdescribed with exemplary reference to computed tomography, it will beappreciated that the invention will also find application in conjunctionwith other medical diagnostic imaging systems that employ other imagingmodalities such as single photon emission computed tomography, positronemission tomography, magnetic resonance imaging, ultrasound imaging, andthe like.

The computed tomography scanner 12 includes an x-ray source 14 and areceiving x-ray detector array 16 oppositely arranged respective to animaging region 20 defined by a rotating gantry 18. A patient to beimaged is arranged on a subject support 22 and inserted into the imagingregion 20. The x-ray source 14 produces a cone-shaped, wedge-shaped, orotherwise-shaped x-ray beam directed into the imaging region 20, wherex-rays are partially absorbed by the patient. The absorption-attenuatedx-ray intensities are measured at the x-ray detector array 16 aftertraversing the imaging region 20, and x-ray absorption data generated bythe detector is stored in an imaging data memory 24.

Preferably, the gantry 18 rotates continuously during the imaging toacquire projection views over at least a 180° angular span. In helicalcomputed tomography imaging, the subject support 22 advances linearlyduring the imaging. The combination of linear movement of the subjectsupport 22 and circular rotation of the gantry 18 produces a helicalorbiting of the x-ray source 14 about the imaging region 20, duringwhich three-dimensional volumetric imaging data is acquired and storedin the imaging data memory 24.

Prior to the imaging, a pair of electrodes 30, 32 are electrically andmechanically connected to a thoracic region of the patient. Preferably,the electrodes 30, 32 are arranged with a substantial portion of thethoracic region therebetween. The electrodes 30, 32 are suitably made ofa material or materials with low x-ray absorption characteristics.Similarly, for other imaging modalities the electrodes are preferablyconstructed to minimize interference with imaging. For magneticresonance imaging applications, for example, the electrodes should bemade of a non-magnetic material or materials.

The electrodes 30, 32 electrically communicate with an impedance meter34 that measures an impedance across the electrodes 30, 32. The measuredimpedance can include resistance, capacitance, inductance, reactance, ora complex impedance combining resistance and reactance. The measuredimpedance varies with spatial separation of the electrodes 30, 32. Thisspatial separation changes as the thorax expands and contracts withinhalation and exhalation, and so the measured impedance carriesrespiratory cycling information. An impedance function is measured,which varies with time as the respiratory cycling progresses.

A respiration monitor 36 receives the measured impedance function andcomputes one or more respiratory characteristics. For example, aperiodicity of the impedance function tracks the respiration rate. Aslope of the impedance function relates to respiration state. If theimpedance is a resistance, the resistance increases with increasingtidal volume of air in the lungs because the path length between theelectrodes 30, 32 increases. Hence, inhalation is indicated by apositive slope of the time-varying resistance function. Exhalation issimilarly indicated by a negative slope of the resistance function. Abreath-hold or other extended interval between inhalation and exhalationis indicated by a generally horizontal slope. Extrema (i.e., peaks andvalleys, or maxima and minima) of the time-varying impedance functionindicate transitions between inhalation and exhalation, and providetemporal markers for estimating respiratory cycle phase.

The one or more respiratory characteristics output by the respirationmonitor 36 can be used in various ways. For example, in one applicationa respiratory cycle binning processor 40 sorts acquired projection viewsby respiratory cycle phase (φ) which is indicated by the respiratorymonitor 36. The respiratory cycle binning processor 40 sorts theprojection views into respiratory cycle phase bins 42, with each binstoring projection views acquired over a selected respiratory cyclephase interval. To meet data sampling requirements, some data may befrom another phase which is similarly configured. A respiratory cyclephase selector 44 selects one or more respiratory phases forreconstruction. A reconstruction processor 46 reconstructs imaging datastored in the selected respiratory cycle phase bins 42 into one or morevolumetric image representations that are stored in an image memory 48.By reconstructing data acquired over a limited respiratory cycle phaseinterval, image motion blurring due to respiration is reduced. A videoprocessor 50 generates a displayable image from the volumetric imagerepresentation, such as a maximum intensity projection, an extractedtwo-dimensional slice, or a three-dimensional rendering, which isdisplayed on a computer 52 or other user interfacing device.

In another application, the respiratory characteristic is used toidentify respiratory activity during imaging data acquisition. An imageregistration processor 60 relatively spatially registers imagerepresentations based on the monitored respiratory activity to correctfor respiration-related motion of an imaged organ of interest. Forexample, portions of the reconstructed image are expanded or contractedto adjust the images to a constant respiratory state. The videoprocessor 50 produces displayable images of the spatially registeredimage representations, which have reduced respiration-related motionblurring due to the image registration processing.

In yet another application, the respiratory characteristic is used totrigger image data acquisition or to perform prospective respiratorygating of the data acquisition. For example, the respiratorycharacteristic can be monitored to detect a breath-hold state. When abreath-hold is detected, an imaging sequence supplied to a computedtomography (CT) scanner controller 64 via the user interface 52 istriggered. In response to the triggering, the controller 64 causes thecomputed tomography scanner 12 to execute the selected imaging sequence.In prospective respiratory gating, the controller 64 initiates intervalsof imaging data acquisition during selected respiratory cycle phaseintervals indicated by the respiration monitor 36. Dose-modulatedimaging synchronized with the respiratory cycle can similarly beperformed.

For cardiac computed tomography imaging, an electrocardiograph 66suitably monitors cardiac cycling simultaneously with monitoring of therespiratory cycle using the electrodes pair 30, 32. In a preferredembodiment, the impedance meter 34 measures a pulse-modulated signalwith a pulse frequency substantially higher than the heart rate. Hence,the impedance measurement signal and the electrocardiographic signal arereadily decoupled by frequency selective filtering. Theelectrocardiograph 66 in the illustrated embodiment gates image dataacquisition. Alternatively, the electrocardiographic information isconveyed to the binning processor 40 to bin the data based on bothcardiac and pulmonary phase.

With reference to FIG. 2, the exemplary impedance meter 34 measures atime-varying resistance R(t) 70. Specifically, a pulse generator 72produces a voltage pulse train applied across the electrodes pair 30,32. The pulse train is preferably has a frequency in the tens ofkilohertz range, with the precise frequency selected to lie withinapplicable frequency allocation bands to avoid generating undesirableradio frequency interference. The frequency of the pulse generator 72 isoptionally adjustable, in which case the applied frequency can beoptimized to maximize the impedance signal. An amplitude of the voltagepulses is typically a few volts or lower. An ammeter 74 measures currentat the pulse train frequency. Optionally, a frequency-selective filter76 removes noise or other interference from the measured current. Animpedance calculator 78 computes the resistance by ratioing appliedvoltage and measured current to compute the time-varying resistance R(t)70.

Those skilled in the art can readily modify the exemplary impedancemeter 34 to suit specific applications or to take advantage of availableelectronic components. For example, a current pulse train can besourced, and voltage measured by voltmeter. Other types of impedancesuch as capacitance, inductance, or complex impedance can be computed bysuitably taking into account phase-shifts between the voltage andcurrent pulses or by otherwise combining the voltage and current data.Moreover, a voltage, current, or other electrical parameter can be usedto characterize respiration. For a steady-state voltage pulse train, theoutput of the ammeter 74 carries respiratory information. Forrespiratory monitoring during magnetic resonance imaging, a lowfrequency or d.c. voltage or current is preferably applied to obviateradio frequency interference concerns.

When using the voltage pulse train input produced by the pulse generator72, it will be appreciated that the electrocardiograph 66 can measure anelectrocardiographic signal 80 simultaneously with measurement of thetime-varying resistance 70 or other time-varying electrical parameter.Using the electrode pair 30, 32 for both respiratory monitoring andcardiac monitoring advantageously reduces imaging delays and simplifiespatient preparation, and further benefits the patient by reducing atotal number of contacting probes. To remove the high-frequency signalcomponents produced by the pulse generator 72, a lowpass filter 82 issuitably applied prior to electrocardiographic measurement. Although aseparate filtering component 82 is shown in FIG. 2, the filter isoptionally omitted if a frequency response of the electrocardiograph 66is such that the electrocardiograph 66 does not respond to signalcomponents at the pulse train frequency.

With reference to FIG. 3, an exemplary embodiment of the respirationmonitor 36 outputs a respiration state 90 which classifies the patientas being in one of an exhalation state, an inhalation state, and abreath-hold or transitional respiratory state. A first derivativeprocessor 94 computes a first derivative of the time-varying resistance70 using an analog differentiator circuit, numerical differentiation,analytic differentiation of a fitted curve or fitted spline segments, orthe like. A threshold processor 96 classifies the first derivativesignal as one of positive, negative, or substantially zero. It will berecognized that a positive first derivative of the measured resistance70 corresponds to an increasing time-varying resistance, which in turncorresponds to an increasing resistance path length that occurs duringinhalation chest expansion. Similarly, a negative first derivative ofthe measured resistance 70 corresponds to a decreasing time-varyingresistance, a decreasing path length, and hence exhalation. Asubstantially zero first derivative of the measured resistance 70corresponds to a constant thoracic volume, which typically indicates anintentional breath-hold by the patient or a fully inhaled or fullyexhaled state.

With continuing reference to FIG. 3, the respiration monitor 36 alsopreferably measures a respiration rate 98. The respiration rate isrelatively low frequency, typically around 12 breaths per minute (˜0.2Hz). In one suitable measurement approach a low pass filter 100, such asa filter with a cutoff frequency f_(c)˜0.5 Hz, filters out higherfrequency components of the time-varying resistance R(t) 70. A frequencyprocessor 102 determines a temporal frequency of the filtered signalcorresponding to the respiration rate 98 using a fast Fourier transform(FFT) analysis, peak detection and counting, or the like.

Other respiratory parameters can similarly be estimated from thetime-varying resistance R(t) 70. For example, a respiratory cycle phaseprocessor 104 estimates a respiratory cycle phase φ(t) 110 based on thetime-varying resistance R(t) 70 and/or time-varying functions extractedtherefrom such as the first derivative generated by the first derivativeprocessor 94 or a second derivative generated by a second derivativeprocessor 106. To reduce computational complexity, it is typicallyadvantageous to differentiate the output of the first derivativeprocessor 94 to obtain the second derivative, as shown in FIG. 3.However, a method that computes the second derivative directly from thetime-varying resistance 70 can also be employed. The respiratory cyclephase φ(t) 110 is suitably estimated by the respiratory cycle phaseprocessor 104 by curve-fitting the time-varying resistance R(t) 70and/or derivatives thereof with a suitable respiratory cycle model.Alternatively, the respiratory cycle phase processor 104 can locatecritical points (such as maxima, minima, or inflection points) of thetime-varying resistance R(t) 70 and/or derivatives thereof thatcorrespond to certain respiratory cycle phases such as transitions frominhalation to exhalation, transitions from exhalation to inhalation, andthe like.

With reference to FIG. 4, another exemplary embodiment of therespiration monitor 36 outputs quantitative values for tidal volume V(t)120 of air in the lungs. Specifically, a patient calibration 122 thatcorrelates tidal volume with resistance between the electrodes 30, 32 isobtained prior to tomographic imaging, for example by measuringresistance values across the electrodes 30, 32 during quantitativespirometric measurement of tidal volumes. Using the patient calibration122, the time-varying resistance R(t) 70 is processed by a tidal volumetransform processor 124 to convert the resistance R(t) 70 toquantitative values of tidal volume V(t) 120.

It will be appreciated that components of the respiration monitor ofFIGS. 3 and 5 are readily combinable. Moreover, other respirationcharacteristics besides the exemplary respiration state, respirationrate, respiration cycle phase, and tidal volume characteristics can becomputed based on the time-varying resistance or other time-varyingelectrical parameter 70. Again, the respiration rate can be fit to amodel and used to predict future respiratory cycles.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A diagnostic imaging system including: a diagnostic imaging scannerthat acquires imaging data of a subject in an examination region; areconstruction processor that reconstructs the acquired imaging datainto an image representation; a pair of electrodes adapted to contact athoracic region of the subject; an electrical meter that measures atime-varying electrical parameter across the electrode pair; and amonitor that extracts a time-varying respiration characteristic from themeasured time-varying electrical parameter.
 2. The imaging system as setforth in claim 1, wherein the time-varying electrical parameter isselected from a group consisting of: a time-varying complex impedance, atime-varying resistance, a time-varying capacitance, a time-varyinginductance, a time-varying current, and a time-varying voltage.
 3. Theimaging system as set forth in claim 1, wherein the diagnostic imagingscanner is a computed tomography scanner.
 4. The imaging system as setforth in claim 1, wherein the electrical meter includes: a voltage pulsegenerator that applies a voltage pulse train to the electrode pair; andan ammeter that measures an electrical current flowing between theelectrode pair responsive to the applied voltage pulse train.
 5. Theimaging system as set forth in claim 1, further including: an imagingcontroller that receives the respiration characteristic and controls thediagnostic imaging scanner based thereon.
 6. The imaging system as setforth in claim 1, wherein the monitor includes: a differentiator thatcomputes a time derivative of the time-varying electrical parameter. 7.The imaging system as set forth in claim 6, wherein the time-varyingelectrical parameter includes a time-varying resistance, thedifferentiator computes a first derivative, and the monitor furtherincludes: a respiration state processor that computes the respirationparameter as one of: inhaling corresponding to a positive timederivative of the time-varying resistance, exhaling corresponding to anegative time derivative of the time-varying resistance, and abreath-hold state corresponding to a substantially zero time derivativeof the time-varying resistance.
 8. The imaging system as set forth inclaim 1, wherein the monitor includes: a respiratory cycle phaseprocessor that estimates a respiratory cycle phase based on thetime-varying electrical parameter.
 9. The imaging system as set forth inclaim 1, wherein the monitor includes: a calibration that correlateselectrical parameter values with a tidal volume of air in lungs of thesubject; and a transform processor that references the calibration totransform the time-varying electrical parameter into a time-varyingtidal volume of air in the lungs.
 10. The imaging system as set forth inclaim 1, further including: an image data binning means for sortingimaging data into respiratory cycle phase bins based on the time-varyingrespiration characteristics, the reconstruction processor reconstructingdata in a selected one or more of the respiratory cycle phase bins. 11.The imaging system as set forth in claim 1, further including: anelectrocardiograph that measures electrocardiographic data of thesubject using at least the pair of electrodes.
 12. The imaging system asset forth in claim 1, wherein a substantial portion of the thoracicregion of the subject is disposed between the contacting electrodes. 13.A medical diagnostic imaging method including: acquiring imaging data ofa medical imaging patient; reconstructing at least a part of theacquired imaging data into an image representation; measuring atime-varying electrical parameter across an electrodes pair during theacquiring of imaging data; and computing a time-varying respirationcharacteristic based on the measured time-varying electrical parameter.14. The method as set forth in claim 13, further including: contacting athoracic region of the patient with the pair of electrodes.
 15. Themethod as set forth in claim 14, wherein the contacting of the thoracicregion with the electrodes pair includes: relatively arranging theelectrodes pair with a substantial portion of the thoracic regiondisposed therebetween.
 16. The method as set forth in claim 13, whereinthe acquiring of imaging data includes: passing x-rays through animaging region; measuring x-ray intensities after passing through theimaging region; and computing x-ray absorption data from the measuredx-ray intensities.
 17. The method as set forth in claim 13, wherein themeasuring of a time-varying electrical parameter includes: applying oneof a voltage and a current to the electrodes pair; measuring the otherof voltage and current responsive to the applying; and computing thetime-varying electrical parameter based on the applied and measuredquantities.
 18. The method as set forth in claim 17, wherein theapplying step includes: applying a pulse train of voltage or currentpulses.
 19. The method as set forth in claim 13, further including:measuring cardiac cycling data using the pair of electrodes.
 20. Themethod as set forth in claim 19, wherein the measuring of cardiaccycling data using the pair of electrodes is performed substantiallysimultaneously with the measuring of a time-varying electrical parameteracross the electrodes pair.
 21. The method as set forth in claim 13,wherein the measuring of a time-varying electrical parameter across theelectrodes pair includes: measuring a time-varying resistance across theelectrodes pair.
 22. The method as set forth in claim 13, wherein thecomputing of a time-varying respiration characteristic from thetime-varying electrical parameter includes: determining a respirationstate based on a temporal slope of the time-varying electricalparameter.
 23. The method as set forth in claim 13, wherein thecomputing of a time-varying respiration characteristic from thetime-varying electrical parameter includes: selecting a respirationstate based on a temporal slope of the time-varying electricalparameters, the respiration state being selected as one of: inhalingcorresponding to a positive temporal slope, exhaling corresponding to anegative temporal slope, and a breath-hold state corresponding to agenerally horizontal slope.
 24. The method as set forth in claim 13,wherein the computing of a time-varying respiration characteristic fromthe time-varying electrical parameter includes: computing a respirationrate proportional to a temporal frequency of the time-varying electricalparameter.
 25. The method as set forth in claim 13, wherein thecomputing of a time-varying respiration characteristic from thetime-varying electrical parameter includes: computing a time-varyingtidal volume function of air in lungs of the patient based on thetime-varying electrical parameter.
 26. The method as set forth in claim13, wherein the computing of a time-varying respiration characteristicfrom the time-varying electrical parameter includes: computing atime-varying respiratory cycle phase function based on the time-varyingelectrical parameter.
 27. The method as set forth in claim 13, furtherincluding: gating the acquiring of imaging data based on the extractedtime-varying respiration characteristic.